دانشنامه آزاد ۴ زبانه / εγκυκλοπαίδεια / licence

Elton پروژه‌ای چندزبانه برای گردآوری دانشنامه‌ای جامع و با محتویات آزاد است

دانشنامه آزاد ۴ زبانه / εγκυκλοπαίδεια / licence

Elton پروژه‌ای چندزبانه برای گردآوری دانشنامه‌ای جامع و با محتویات آزاد است

Television

"TV" redirects here. For other uses, see TV (disambiguation).
"Telly" redirects here. For the character Telly on Sesame Street, see Telly Monster.
"Idiot box" redirects here. For other uses, see Idiot box (disambiguation).
For the band, see Television (band).
Braun HF 1, Germany, 1958.
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Braun HF 1, Germany, 1958.
Television Portal

Television is a telecommunication system for broadcasting and receiving moving pictures and sound over a distance. The term has come to refer to all the aspects of television from the television set to the programming and transmission. The word is derived from mixed Latin and Greek roots, meaning "far seeing" (Greek "tele," meaning far, and Latin "visus," meaning seeing).

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Origins

Main article: History of television

The origins of what would become today's television system are traced back as far as the discovery of the photoconductivity of the element selenium by Willoughby Smith in 1873, and the invention of a scanning disk by Paul Nipkow in 1884. All practical television systems use the fundamental idea of scanning an image to produce a time series signal representation. That representation is then transmitted to a device to reverse the scanning process. The final device, the television, relies on the human eye to integrate the result into a coherent image again.

While electromechanical techniques were developed prior to World War II, most notably by Charles Francis Jenkins and John Logie Baird, all-electronic television systems relied on the inventions of Philo Taylor Farnsworth, Vladimir Zworykin and others to produce a system suitable for mass distribution of television programming. Commercial broadcast programming, following years of experimental broadcasts seen only in a few specially-equipped homes, occurred in both the United States and the United Kingdom before World War II.

The first television broadcasts with a modern level of definition (240+ lines) were made in England in 1936. Television did not become commonplace in United States homes until the middle 1950s. While North American over-the-air broadcasting was originally free of direct marginal cost to the consumer (i.e., cost in excess of acquisition and upkeep of the hardware) and broadcasters were compensated primarily by receipt of advertising revenue, increasingly television consumers obtain their programming by subscription to cable television systems or direct-to-home satellite transmissions. In the United Kingdom, on the other hand, the owner of each television must pay a licence fee annually which is used to support the British Broadcasting Corporation.

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Technology

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Elements of a television system

The elements of a simple television system are:

  • An image source - this may be a camera for live pick-up of images or a flying spot scanner for transmission of films
  • A sound source.
  • A transmitter, which modulates one or more television signals with both picture and sound information for transmission.
  • A receiver (television) which recovers the picture and sound signals from the television broadcast.
  • A display device, which turns the electrical signals into visible light and audible sound.

Practical television systems include equipment for selecting different image sources, mixing images from several sources at once, insertion of pre-recorded video signals, synchronizing signals from many sources, and direct image generation by computer for such purposes as station identification. Transmission may be over the air from land-based transmitters, over metallic or optical cables, or by radio from synchronous satellites. Digital systems may be inserted anywhere in the chain to provide better image transmission quality, reduction in transmission bandwidth, special effects, or security of transmission from theft by non-subscribers.


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Display technology

Thanks to advances in display technology, there are now several kinds of video displays used in modern TV sets:

  • CRT: The most common displays are the ubiquitous direct-view CRTs for up to 40in (100cm) (in 4:3) and 46in (115cm) (in 16:9) diagonally. These are still the least expensive, and are a refined technology that can still provide the best overall picture quality. As they do not have a fixed native resolution, in some cases they are also capable of displaying sources with a variety of different resolutions at the best possible image quality. The frame rate or refresh rate of a typical NTSC format CRT TV is 60 Hz, and for the PAL format, it's 50 Hz. A typical NTSC broadcast signal's visible portion has an equivalent resolution of about 640x480 pixels. It actually could be slightly higher than that, but the Vertical Blanking Interval, or VBI, allows other signals to be carried along with the broadcast.
  • Rear projection: Most big-screen TVs (up to over 100 inch (254 cm)) use projection technology. Three types of projection systems are used in projection TVs: CRT-based, LCD-based, and DLP(reflective micromirror chip) -based. Projection television has been commercially available since the 1970s, but could not match the image sharpness of the CRT; current models are vastly improved, and offer a cost-effective large-screen display. A variation is a video projector, using similar technology, which projects onto a screen.
  • Flat panel LCD or plasma: Modern advances have brought flat panels to TV that use active matrix LCD or plasma display technology. Flat panel LCDs and plasma displays are as little as 4in (10cm) thick and can be hung on a wall like a picture or put over a pedestal. Some models can also be used as computer monitors.
See also: Liquid crystal display television
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Signal connections

The number of ways to connect a video device to a television has increased over the years:

  • DVI - a 17 to 29-pin connector that carries digital video signals, designed to carry HDTV but also used in current DVD players and latest digital displays. Copy protection is available using HDCP.
  • Component video - three separate RCA jacks (colored red, green and blue) carry three analog video signals, one brightness (luminance) and two colors (chromas), and is usually referred to as "Y, B-Y, R-Y", "Y Cr Cb" (interlaced) or "Y Pr Pb" (progressive), or YUV. Audio is not carried on this cable. This connection provides for picture quality superior to S-Video and is typically used in home theater for DVDs, satellite and analogue HDTV; less common in Europe but is starting to become more widely available.
  • SCART - a large 21 pin connector that may carry analog signals consisting of: one video signal composite video; or two video signals S-Video; or for picture quality similar to component video, three signals of separate red, green and blue or RGB; or for best picture quality, four video signals of separate red, green, blue and sync or RGBS; plus right and left line-level audio channels; along with a number of control signals including an aspect-ratio flag (e.g. widescreen). This system has been standard in Europe since mid-1980s for all consumer electronics, which meant that RGBS was available on even the earliest PAL DVD players and satellite receivers. Japan uses a 21 pin RGB connector which is visually similar to SCART but with different pin configurations. This connector is not used in the U.S.
  • S-Video - small round connector with two separate analog video signals, one carrying brightness (luminance), the other carrying color (chroma). Also referred to as Y/C video. Provides most of the benefit of component video, with slightly less color fidelity. Use started in the 1980s for S-VHS, Hi-8, and early NTSC DVD players to relay high quality video before component was available. This will sometimes, completely incorrectly, be referred to as an S-VHS connector. Audio is not carried on this cable.
  • Composite video - The most common form of connecting external devices, putting all the analog video information into one signal. Most televisions provide this option with a yellow RCA jack or occasionally a BNC connector. Audio is not carried on this cable, though two separate cables with similar red and white RCA jacks for right and left line-level audio are commonly bonded to composite video cables.
  • Coaxial RF - All audio channels and picture components are transmitted through one coaxial cable and modulated on a radio frequency. Most TVs manufactured since the 1970s provide a coaxial connection, and this is the type of cable typically used for cable television. Although still found on VHS tape-players, most modern DVD players and other video devices no longer supply an RF output, so very old TV sets made before composite video jacks became commonplace will need a modulator device. NTSC sets use a 75 ohm F-connector; most PAL sets use a 50 ohm Belling Lee. Most set-top TV antennas have a 300 ohm impedance, so to connect them to a coaxial input requires an inexpensive matching transformer to avoid signal degradation.
  • 300 ohm twin-lead - The predecessor to coaxial cable, generally a flat insulated cable with a pair of wires separated by 0.5 inch, found on NTSC television sets from 1940 to about 1985, and originally used to connect rabbit ears to a receiver. Connection to the set was by connecting the wire to a pair of screws on the back of the television set. Nominal impedance was 300 ohms; connecting an older set to cable or VCRs requires an inexpensive matching transformer to avoid signal degradation due to impedance mismatch. Twin-lead wiring is sensitive to nearby metal objects. Long runs must be properly supported away from metal objects and should be mounted with a loose twist in the cable - see unshielded twisted pair cables which operate by the same principle as properly installed twin-lead.
  • Fiber optic - The latest in connections and only on extremly new and high quality TVs, fiber optics use a laser to transmit data along a glass fiber. Used for sound and/or video, can be found on newer TVs, high end video editing systems, as well as in high end computer systems. Can be referred to as S/PDIF digital-audio format.
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Aspect ratios

Aspect ratio refers to the ratio of the horizontal to vertical measurements of a television's picture. Mechanically scanned television as first demonstrated by John Logie Baird in 1926 used a 7:3 vertical aspect ratio, oriented for the head and shoulders of a single person in close-up.

Most of the early electronic TV systems from the mid-1930s onward shared the same aspect ratio of 4:3 which was chosen to match the Academy Ratio used in cinema films at the time. This ratio was also square enough to be conveniently viewed on round cathode-ray tubes (CRTs), which were all that could be produced given the manufacturing technology of the time. (Today's CRT technology allows the manufacture of much wider tubes, and the flat-screen technologies which are becoming steadily more popular have no aspect ratio limitations at all.) The BBC's television service used a more squarish 5:4 ratio from 1936 to 3 April 1950, when it too switched to a 4:3 ratio.

In the 1950s, movie studios moved towards widescreen aspect ratios such as CinemaScope in an effort to distance their product from television. Although this was initially just a gimmick, widescreen is still the format of choice today and square aspect ratio movies are rare. Some people argue that widescreen is actually a disadvantage when showing objects that are tall instead of panoramic, others say that natural vision is more panoramic than tall, and therefore widescreen is easier on the eye.

The switch to digital television systems has been used as an opportunity to change the standard television picture format from the old ratio of 4:3 (approximately 1.33:1) to an aspect ratio of 16:9 (approximately 1.78:1). This enables TV to get closer to the aspect ratio of modern widescreen movies, which range from 1.78:1 through 1.85:1 to 2.35:1. There are two methods for transporting widescreen content, the better of which uses what is called anamorphic widescreen format. This format is very similar to the technique used to fit a widescreen movie frame inside a 1.33:1 35mm film frame. The image is compressed horizontally when recorded, then expanded again when played back. The anamorphic widescreen 16:9 format was first introduced via European PAL-Plus television broadcasts and then later on "widescreen" DVDs; the ATSC HDTV system uses straight widescreen format, no horizontal compression or expansion is used.

Recently "widescreen" has spread from television to computing where both desktop and laptop computers are commonly equipped with widescreen displays. There are some complaints about distortions of movie picture ratio due to some DVD playback software not taking account of aspect ratios; but this will subside as the DVD playback software matures. Furthermore, computer and laptop widescreen displays are in the 16:10 aspect ratio both physically in size and in pixel counts, and not in 16:9 of consumer televisions, leading to further complexity. This was a result of widescreen computer display engineers' uninformed assumption that people viewing 16:9 content on their computer would prefer that an area of the screen be reserved for playback controls or subtitles, as opposed to viewing content full-screen.

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Aspect ratio incompatibility

The television industry's changing of aspect ratios is not without teething difficulties, and can present a considerable problem.

Displaying a widescreen aspect (rectangular) image on a conventional aspect (square) display can be shown:

  • in "letterbox" format, with black horizontal bars at the top and bottom
  • with part of the image being cropped, usually the extreme left and right of the image being cut off (or in "pan and scan", parts selected by an operator)
  • with the image horizontally compressed

A conventional aspect (square) image on a widescreen aspect (rectangular) display can be shown:

  • in "pillar box" format, with black vertical bars to the left and right
  • with upper and lower portions of the image cut off (or in "tilt and scan", parts selected by an operator)
  • with the image horizontally distorted

A common compromise is to shoot or create material at an aspect ratio of 14:9, and to lose some image at each side for 4:3 presentation, and some image at top and bottom for 16:9 presentation. In recent years, the cinematographic process known as Super 35 (championed by James Cameron) has been used to film a number of major movies such as Titanic, Legally Blonde, Austin Powers, and Crouching Tiger, Hidden Dragon (see also: List of top-grossing films shot in Super 35). This process results in a camera-negative which can then be used to create both wide-screen theatrical prints, and standard "full frame" releases for television/VHS/DVD which avoid the need for either "letterboxing" or the severe loss of information caused by conventional "pan-and-scan" cropping.

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Sound

Further information: NICAM
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Television add-ons

Today there are many television add-ons including Video Game Consoles, VCRs, Set-top boxes for Cable and Satellite reception, DVD players, or Digital Video Recorders (including personal video recorders, PVRs). The add-on market continues to grow as new technologies are developed.

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New developments

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Geographical usage

Main article: Geographical usage of television
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Content

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Advertising

Since their inception in the USA in 1940, TV commercials have become one of the most effective, most pervasive, and most popular methods of selling products of many sorts, especially consumer goods. U.S. advertising rates are determined primarily by Nielsen Ratings.

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Programming

Getting TV programming shown to the public can happen in many different ways. After production the next step is to market and deliver the product to whatever markets are open to using it. This typically happens on two levels:

  1. Original Run or First Run - a producer creates a program of one or multiple episodes and shows it on a station or network which has either paid for the production itself or to which a license has been granted by the producers to do the same.
  2. Syndication - this is the terminology rather broadly used to describe secondary programming usages (beyond original run). It includes secondary runs in the country of first issue, but also international usage which may or may not be managed by the originating producer. In many cases other companies, TV stations or individuals are engaged to do the syndication work, in other words to sell the product into the markets they are allowed to sell into by contract from the copyright holders, in most cases the producers.

In most countries, the first wave occurs primarily on free-to-air (FTA) television, while the second wave happens on subscription TV and in other countries. In the U.S. however, the first wave occurs on the FTA networks and subscription services, and the second wave travels via all means of distribution.

First run programming is increasing on subscription services outside the U.S., but few domestically produced programs are syndicated on domestic FTA elsewhere. This practice is increasing however, generally on digital-only FTA channels, or with subscriber-only first run material appearing on FTA.

Unlike the U.S., repeat FTA screenings of a FTA network program almost only occur on that network. Also, affiliates rarely buy or produce non-network programming that isn't intensely local.

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Social aspects

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Alleged dangers

Paralleling television's growing primacy in family life and society, an increasingly vocal chorus of legislators, scientists and parents are raising objections to the uncritical acceptance of the medium. For example, the Swedish government imposed a total ban on advertising to children under twelve in 1991 (see advertising). In the U.S., the National Institute on Media and the Family (not a government agency) points out that U.S. children watch an average of 25 hours of television per week and features studies showing it interferes with the educational and maturational process.

Fifty years of research on the impact of television on children's emotional and social development (Norma Pecora, John P. Murray, & Ellen A. Wartella, Children and Television: 50 Years of Research, published by Erlbaum Press, June, 2006) demonstrate that there are clear and lasting effects of viewing violence. In a recent study (February, 2006) published in the journal Media Psychology, volume 8, number 1, pages 25-37, the research team demonstrated that the brain activation patterns of children viewing violence show that children are aroused by the violence (increased heart rates), demonstrate fear (activation of the amygdala-the fight or flight sensor in the brain) in response to the video violence, and store the observed violence in an area of the brain (the Posterior Cingulate) that is reserved for long-term memory of traumatic events.

A 23 February 2002 article in Scientific American suggested that compulsive television watching was no different from any other addiction, a finding backed up by reports of withdrawal symptoms among families forced by circumstance to cease watching.

A longitudinal study in New Zealand involving 1000 people (from childhood to 26 years of age) demonstrated that "television viewing in childhood and adolescence is associated with poor educational achievement by 26 years of age". In other words, the more the child watched television, the less likely he or she was to finish school and enroll in a university.

The changes in TV equipment and programming has been noted as one of the largest changes known to people because it was vital to people in the past as well as in the present.

In Iceland, television broadcasting hours were restricted until 1984, with no television programs being broadcast on Thursday, or during the whole of July.

Television's biggest social aspect is the fact that it allows users to instantly view content that may be occuring far away from where they are. Television has been a major contributor in the process of globalization. The Earth's residents can be linked by the programming they watch; but like the internet, television has been taken advantage of as a perfect medium for advertising and the spreading of thoughts and ideas.

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Technology trends

In its infancy, television was an ephemeral medium. Fans of regular shows planned their schedules so that they could be available to watch their shows at their time of broadcast. The term appointment television was coined by marketers to describe this kind of attachment.

The viewership's dependence on schedule lessened with the invention of programmable video recorders, such as the Videocassette recorder and the Digital video recorder. Consumers could watch programs on their own schedule once they were broadcast and recorded. Television service providers also offer video on demand, a set of programs which could be watched at any time.

Both mobile phone networks and the internet are capable of carrying video streams. There is already a fair amount of internet TV, while mobile phone TV is planned to become mainstream, if it can be effectively sold, early in 2006.

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Suitability for audience

Almost since the medium's inception there have been charges that some programming is, in one way or another, inappropriate, offensive or indecent. In addition, a lot of television has been charged with presenting propaganda, political or otherwise, and been pitched at a low intellectual level.

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Further reading

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References

  • David E. Fisher and Marshall J. Fisher, Tube, the Invention of Television, Counterpoint, Washington D.C. USA, (1996) ISBN 1887178171
  • Alex J. Muscat and Anthony J. Lakkis, The History of Television 1942 to 2000, McFarland, Washington D.C. USA, (2001) ISBN 0-7864-1852-4
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See also

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External links

Find more information on Television by searching Wikipedia's sister projects:

Dictionary definitions from Wiktionary
Textbooks from Wikibooks
Quotations from Wikiquote
Source texts from Wikisource
Images and media from Commons
News stories from Wikinews

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